1. Field of the Invention
This invention relates to the art of electrophotography and more particularly, to a method for fabricating photosensitive materials for electrophotography which make use of organic photosensitive compounds and are particularly suitable for use in electrophotography for positive charge systems. The invention also relates to photosensitive materials which are particularly resistant to ozone with high durability.
2. Description of the Prior Art
Electrophotographic photosensitive materials can be broadly classified into two groups. One group makes use of inorganic photoconductors as a photosensitive material. Typical of the inorganic photoconductors are selenium, zinc oxide, titanium oxide, cadmium sulfide and the like. Another group makes use of organic photoconductors such as phthalocyanine pigments, disazo pigments and the like.
In the photosensitive materials using the inorganic photoconductors, the thermal stability and durability are not necessarily satisfactory. In addition, some inorganic photoconductors are disadvantageous in the toxicity thereof, presenting problems on fabrication and handling.
On the other hand, the photosensitive materials using organic photoconductors have a number of advantages over inorganic photosensitive compounds, including the ease in preparation of a variety of compounds exhibiting high sensitivity at different wavelengths depending on the molecular design, little or no ecological problem, and good productivity and economy. Although the problems hitherto involved in organic photosensitive materials include those of durability and sensitivity, these characteristic properties have been remarkably improved at present. Some organic photoconductors have now been in use as main photosensitive materials for electrophotography.
Known organic photosensitive materials usually have a double-layer structure which includes a charge generation layer capable of absorbing light to generate carriers and a charge transport layer wherein the generated carriers are transported. Known materials used to form the charge generation layer include perylene compounds, various phthalocyanine compounds, thia pyrylium compounds, anthanthrone compounds, squalilium compounds, bisazo compounds, trisazo pigments, azulenium compounds and the like.
On the other hand, the materials used to form the charge transport layer include various types of hydrazone compounds, oxazole compounds, triphenylmethane compounds, arylamine compounds and the like.
There is now a high demand of photosensitive materials for recording such as by laser printers wherein the organic photosensitive compounds indicated above are used in a near ultraviolet range corresponding to semiconductor laser beams with a wavelength range of from 780 to 830 nm. Accordingly, organic photosensitive compounds having high sensitivity in the above-indicated near ultraviolet range have been extensively studied and developed. In view of the sensitivity in the above UV range, organic photosensitive compounds are more advantageous than inorganic photosensitive metals or compounds.
The organic photosensitive compounds are usually employed in combination with binder resins and applied onto substrates, such as drums, belts and the like, by relatively simple coating techniques. Examples of the binder resins used for this purpose include polyester resins, polycarbonate resins, acrylic resins, acryl-styrene resins and the like. In general, with the double-layer structure, the charge generation layer is coated in a thickness of several micrometers in order to attain high sensitivity and the charge transport layer is applied in a thickness of several tens of micrometers. From the standpoint of the physical strength and the printing resistance, the charge generation layer should generally be formed directly on the substrate and the charge transport layer is formed as a surface layer. In this arrangement, charge transport compounds which are now in use are only those which act by movement of positive holes. Thus, the known photosensitive materials of the double-layer structure are of the negative charge type.
The negative charge systems, however, have several disadvantages: (1) negative charges used for charging attack oxygen in air into ozone; (2) charging does not proceed satisfactorily; (3) the system is apt to be influenced by surface properties of a substrate such as a drum. Ozone presents the problem that not only ozone is harmful to human bodies, but also it often reacts with organic photosensitive compounds to shorten the life of the photosensitive materials.
In order to solve the above problems, organic photosensitive materials of the positive charge type have been extensively studied. In order to realize the positive charge systems, attempts have been heretofore made including (1) reversed double-layer structures wherein the charge generation layer and the charge transport layer are reversed to the case of the negative charge type; (2) single-layer structures wherein various types of charge generation compounds and charge transport compounds are dispersed in binder resins; and (3) a single-layer structure wherein copper phthalocyanine is dispersed in polymers.
However, the reversed double-layer structure involves the problems similar to the negative charge system, i.e. complicated fabrication processes and the separation of the two layers. In addition, the charge generation layer, which has to be substantially thin, is placed on the surface of the photosensitive material with attendant problems such as reduction in the printing resistance and a poor life characteristic.
On the other hand, the photosensitive materials having the single-layer structure as in (2) and (3) above which are of the positive charge type are inferior to the double-layer structure photosensitive materials with respect to the sensitivity and charge characteristics, i.e. the materials are less likely to be charged, and a great residual potential. The reason why the sensitivity is poorer is that the generation and transport of charges take place randomly in the single layer. Thus, the photosensitive materials having the single-layer structure has the problem to solve when used in practical applications. It will be noted, however, that the single structure as in (2) and (3) above is advantageous in that when the photosensitive material is worn, it does not result immediately in a lowering of printing resistance provided that the charge generation and transport compounds are uniformly dispersed. In addition, the single-layer structure is easier in fabrications than double-layer structures. The drawbacks of the single-layer structure such as the sensitivity, charge characteristics and residual potential, are considered to result from a poor ozone resistance.
It should be noted that organic photosensitive materials of the positive charge type having a single-layer structure or a double-layer structure have been already proposed by the present applicant, for example, in U.S. patent application Ser. No. 551,538 (European Patent Application No. 90.307677.6).